1. Field of the Invention
The present invention relates to a displacement measuring instrument for measuring a dimension or the like of an object to be measured based on an axial displacement of a spindle.
2. Description of Related Art
There have been known micrometers for measuring a dimension or the like of an object to be measured based on an axial displacement of a spindle, some of which include a constant pressure mechanism allowing measurement with a substantially constant measurement force against the object (See Patent Literature 1: JP-A-2010-32365).
Specifically, such an arrangement includes: a stationary sleeve fixed to a body; an axially movable spindle screwed to the stationary sleeve; a thimble rotatably mounted on the outer circumference of the stationary sleeve and having an outer end coupled to the outer end of the spindle; an operation sleeve fitted over a range from the outer circumference of the thimble to the outer end of the spindle; and a constant pressure mechanism interposed between the outer end of the operation sleeve and the outer end of the spindle and freely rotated relative to the spindle when a predetermined or more load is applied to the spindle.
In a micrometer having the above arrangement, the operation sleeve is rotated to cause a forward movement of the spindle in measurement. When a distal end of the spindle comes into contact with the object and the predetermined or more load is applied to the spindle, the constant pressure mechanism is actuated to allow a free rotation of the operation sleeve, so that the object can be measured with a constant measurement force.
Some measurers, however, may apply to the operation sleeve a force other than the rotation force, such as a twisting force or a radial force. If so, the force, such as a twisting force or a radial force, applied to the operation sleeve is directly transmitted to the spindle via the thimble, increasing a friction force at a thread (male thread portion) defined in the spindle and hampering a smooth rotation of the spindle. This results in a variation in the measurement force and, consequently, in measurement values.